The present invention generally relates to fuel cell fabrication techniques and, more particularly, to fabrication of solid electrolyte planar fuel cells.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant to generate a direct current. A fuel cell typically includes a cathode material, an electrolyte material, and an anode material. The electrolyte is a non-porous material sandwiched between the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The electrical interconnect also provides for passageways which allow oxidant fluid to flow past the cathode and fuel fluid to flow past the anode, while keeping these fluids separated. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are typically gases and are continuously passed through separate passageways. Electrochemical conversion occurs at or near the three-phase boundaries of each electrode (cathode and anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Fuel cells with solid electrolytes are the most promising technologies for power generation. Solid electrolytes are either ion conducting ceramic or polymer membranes. In the former instance, the electrolyte is typically made of a ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. With such an electrolyte, the anode is oftentimes made of nickel/NSZ cermet and the cathode is oftentimes made of doped lanthanum manganite.
Perhaps the most advanced construction with ceramic membranes is the tubular solid oxide fuel cell based on cubic zirconia electrolyte. The tubular construction can be assembled into relatively large units without seals and this is its biggest engineering advantage. However, tubular solid oxide fuel cells are fabricated by electrochemical vapor deposition processes, which are slow and costly. The tubular geometry of these fuel cells also limits the specific power density, both on weight and volume bases, to low values. The electron conduction paths are also long and lead to high energy losses due to internal resistance heating. For these reasons, other constructions based on planar cells are actively being pursued.
One alternative of the planar stack construction resembles a cross-flow heat exchanger in a cubic configuration. The planar cross flow fuel cell is built from alternating flat single cell membranes (which are tri-layer anode/electrolyte/cathode structures) and bipolar plates (which conduct current from cell to cell and provide channels for gas flow into a cubic structure or stack). The bipolar plates are oftentimes made of suitable metallic materials. The cross-flow stack is manifolded externally on four faces for fuel and oxidant gas management.
The cross-flow or cubic design, however, requires extensive sealing, both in terms of the number of seal interfaces and the linear size of such interfaces. The latter increases with the stack footprint and leads to serious problems if the metal and ceramic cell parts do not have closely matched thermal expansion coefficients. A significant mismatch in the thermal expansion coefficients leads to thermal stresses that can cause catastrophic failure on cool down from the stack operating temperature.
Internally manifolded radial stack designs require substantially less glass based sealing than the cross-flow design, especially if the required gas streams are introduced into the stack via a central, dual cavity manifold. Advanced radial stack designs have reduced the number of required glass seals to two per cell and one per separator plate, but two of these seals have to be made during stack assembly, are blind seals, and their integrity cannot be inspected and repaired.
The physical integrity and mechanical reliability of sealing cross-flow or radial stacks with glass based sealants is not adequate at the present time. Therefore, in the fabrication of solid oxide fuel cell (SOFC) stacks one of the outstanding material issues is sealants. The sealants for fuel cell stacks must meet several, often competing, property requirements. One of the required properties is that the sealant must have a relatively high coefficient of thermal expansion (CTE) to match that of the stack components such as cells, interconnects and manifold materials. Such stack components typically have CTEs ranging from about 9 to 15xc3x9710xe2x88x926/xc2x0 C. at 800xc2x0 C. Another required property involves the softening point of the sealant. The sealant should have a relatively low softening point (i.e., desirably about 400 to 700xc2x0 C.) for thermal stress relief during system shut-down and start-up. Further, viscosity of the sealant should be within a desired range (i.e., desirably about 103 to 108 poise) at the sealing temperature. The sealant should be fluid enough to seal gaps at the sealing temperature while it should be viscous enough at the SOFC operating temperature (700-900xc2x0 C.) so that gaps are kept sealed under gas pressure differentials. Finally, the sealant must be both thermally stable and chemically stable (negligible weight loss and minimum reaction with stack and manifold materials) in the SOFC operating environments and conditions.
These properties are often very difficult to satisfy using currently available sealants. Of the above given properties, the high CTE and gap-holding capacity are the most difficult to satisfy. Although some commercially available silica based glasses may come close to meeting most of these properties, they generally fall short of meeting the high CTE and gap-holding capacity. These silica-based glasses often fail when an interconnect is made of an high CTE material because such glasses are not able to produce a reliable seal as the thermal expansion mismatch between these two materials leads to part distortion and cracking. Adding to this, it is also known that such glasses cannot seal relatively large gaps, e.g., larger than 1 mm.
As can be seen, there is a need for a better sealant material that judiciously balances all the required properties. In particular, there is a need for a sealant material which has low enough initial viscosity and gap filling capacity as well as pressure containment characteristics to achieve sealing.
In one aspect of the present invention, a process for sealing a fuel cell stack comprises forming a sealant mixture paste, applying the sealant mixture paste to a selected sealing location of the fuel cell stack and transforming the sealant mixture paste into a sealant mixture material to seal the selected sealing locations. The sealant mixture paste is formed by mixing a glass precursor and a composite microstructure forming agent. The sealant mixture material comprises a glass matrix phase and a reinforcing phase.
In another aspect of the present invention, a sealant material for sealing a fuel cell stack comprises a glass matrix phase and a plurality of elongated grains of a reinforcing phase distributed into the glass matrix phase. The elongated grains are single crystals that are formed as a result of a reaction between a glass precursor material and a composite structure forming material selected from the group consisting of barium titanate, strontium titanate, calcium titanate, and mixtures thereof.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.